VCE Physics Question Thread!

[Zealous]

I know this is super simple, but I'm getting really confused looking at these solutions for a prac exam. If you are in an elevator going down at a constant speed and then you slow down to a stop, what is the direction of the acceleration while you are stopping? Hopefully that is worded okay enough so it's not confusing.

Thanks

EDIT: Another question, do magnetic field lines ever cross if they come from the same magnet. Like for example, do the magnetic field lines of a simple bar magnet ever cross each other?

EDIT 2: For the attached image, can someone answer the question. The solutions aren't specific. They just talk about Lenz's law, not specifically why the current flows from A to B. How do we know when we can just be vague when answering the question? Are we not actually able to find out the direction of the current on our own?

1. Upwards. The normal force from the ground will push up on you more than gravity pushes down on you, so you will accelerate upwards to counter your downwards velocity.

2. Nope. Field lines shouldn't touch at all.

3. The solutions are probably not very good. If the question asks to explain why it flows in that direction, I'd be looking to apply Lenz's law to the situation, not just state it.

[Floatzel98]

How do we do question 2 of the attached. I understand we have to use , but I have no idea how to find the force with this information.The solutions just state that the force can be found as 0.005N but I have no idea where they get it from

EDIT: Sorry for so many questions, these exams I'm doing don't have very good solutions. If we needed to draw the normal reaction force for a car do we need to draw two normal forces, for each wheel? I attached the picture.

[odeaa]

How do we do question 2 of the attached. I understand we have to use , but I have no idea how to find the force with this information.The solutions just state that the force can be found as 0.005N but I have no idea where they get it from

EDIT: Sorry for so many questions, these exams I'm doing don't have very good solutions. If we needed to draw the normal reaction force for a car do we need to draw two normal forces, for each wheel? I attached the picture.

Dude I was stuck on that exact same question yesterday, 2012 Insight yeah?

Makes absolutely no sense where they got the force from, I have asked multiple people and I think it might just be a mistake

[Floatzel98]

Dude I was stuck on that exact same question yesterday, 2012 Insight yeah?

Makes absolutely no sense where they got the force from, I have asked multiple people and I think it might just be a mistake

Yeah, 2012 Insight. Hopefully it was just a mistake. I have been sitting here for ages wondering how they got it. I guess I'll just write it off as a mistake for now then.

[Floatzel98]

I just did a question that gave us the resistance of a wire per kilometer and the total distance between the two power stations (this is for a power transmission question). We had to find the power loss in the lines and the solutions said to find the total resistance we had to double the distance because the wires needed to go both ways to complete the circuit. That does make sense but wouldn't the power loss just be effected by the resistance of the transmitted wire? Why does the total resistance matter? The way I'm imagining it is like having two resistors in series and recording the voltage drop over the first resistor. The voltage drop over both resistors doesn't matter until you get up to it, which is the case here. It flows across the first wire until it reaches the transformer and then will never need to come back through the transformer again.

Also on the same thought, can a transformer have input voltages on either side? I know that doesn't really make sense, but from the solutions to my above problem it's making me think that power can be transmitted back through the lines and go back to the initial step up transformer.

One more question, when we have to draw de broglie wavelengths around atoms what is the difference between saying an electron in the third excited state or the n = 4 quantum number?

Thanks

[zsteve]

@FloatZel98: You need to double the resistance. In a way, you're right in saying that the voltage drop due to the return wire occurs 'on the way back'. However, note that the resistance for the return wires DOES change the voltage drop across EVERY COMPONENT IN THE CIRCUIT. This is because it affects total resistance and hence reduces the current flowing through what is effectively a series circuit. So in response to "The voltage drop over both resistors doesn't matter until you get up to it, which is the case here", yes that is right, but the voltage drop over everything is different because the extra resistance at the 'end' of the current's path.

It flows across the first wire until it reaches the transformer and then will never need to come back through the transformer again.

Current WILL have to flow through the transformer in the reverse direction... it will induce the current in the reverse direction that is. Current does flow in both directions over the transformers (otherwise you violate conservation of charge)
A transformer can take input voltages from either side. There is nothing in its mechanization which precludes this, that is, for the fundamental transformer concept.
3rd excited state <=> n=4 level, so 4 wavelengths around the atomic orbital. No difference, just n=1 is ground state.

[Floatzel98]

@FloatZel98: You need to double the resistance. In a way, you're right in saying that the voltage drop due to the return wire occurs 'on the way back'. However, note that the resistance for the return wires DOES change the voltage drop across EVERY COMPONENT IN THE CIRCUIT. This is because it affects total resistance and hence reduces the current flowing through what is effectively a series circuit. So in response to "The voltage drop over both resistors doesn't matter until you get up to it, which is the case here", yes that is right, but the voltage drop over everything is different because the extra resistance at the 'end' of the current's path.Current WILL have to flow through the transformer in the reverse direction... it will induce the current in the reverse direction that is. Current does flow in both directions over the transformers (otherwise you violate conservation of charge)
A transformer can take input voltages from either side. There is nothing in its mechanization which precludes this, that is, for the fundamental transformer concept.
3rd excited state <=> n=4 level, so 4 wavelengths around the atomic orbital. No difference, just n=1 is ground state.

Thanks so much! Clears everything up

[dankfrank420]

Anyone got any strategies for those astrophysics questions?

It's not that they're difficult, its just that the lengthy calculations mean I always mess up.

[Floatzel98]

Anyone got any strategies for those astrophysics questions?

It's not that they're difficult, its just that the lengthy calculations mean I always mess up.

Storing values of some of the constants can be very helpful. That way you make sure you don't input any numbers incorrectly. There isn't much else you can do besides double checking what you are writing out and putting in your calculator.

I have a question again. With questions about thermistors (or LDR's) that ask about whether you should increase/decrease the resistance of the resistor in the circuit (or any other variation), is it okay to use a calculation as part of your answer.

[odeaa]

I have a question again. With questions about thermistors (or LDR's) that ask about whether you should increase/decrease the resistance of the resistor in the circuit (or any other variation), is it okay to use a calculation as part of your answer.

They can't penalise you for it if it's correct
I usually don't bother but for 3 marks might be better of using a calculation just to be on the safe side

[odeaa]

So the only thing I still don't really understand in the course is modulation and transmission of waves (in electronics). I don't really understand amplitude modulation and frequency modulation (which I don't think is in the course though?). A lot of the questions about carrier waves transferring analog information talk about the carrier wave being in time variation of the intensity or something. I'm just not really understanding it at all.

Any kind of explanation would be great. Thanks

sorry it took so long, here ya go

[dankfrank420]

When using the radius of orbit, we include the radius of the mass we're orbiting right? (eg. r = radius of earth + height of satellite above Earths surface).

[Zealous]

When using the radius of orbit, we include the radius of the mass we're orbiting right? (eg. r = radius of earth + height of satellite above Earths surface).

Yep! Make sure to include the altitude in all your calculations.

[Adequace]

In forces, does it matter if I draw contact forces from the center of a body or would I lose marks?

[zsteve]

Just came back from a revision lecture today. Yes, you do need to draw from point of application (something I've been avoiding all year LOL).
If they give you an actual diagram (i.e. car) -
 - FOUR normal forces, from the GROUND/PLANE on the TYRES
 - ONE weight force from CENTRE OF MASS.
If you get a block
 - Draw everything from centre of mass, this is a free body diagram.

Unlike Spesh